1. Field of the Invention (Technical Field)
The present invention relates to an improvement to wavelength modulation spectroscopy.
2. Background Art
Wavelength modulation spectroscopy (WMS) is a form of optical absorption spectroscopy that allows detection of small optical absorbances. The technique is effective because absorption measurements are shifted from frequencies near DC, where light sources are noisy, to high frequencies where shot-noise-limited absorption measurements are possible. This shift in detection band can improve measurement sensitivity by three to five orders of magnitude.
WMS is usually implemented with continuously tunable lasers such as diode lasers. Typically, the wavelength of the light source is modulated by a small amount about an absorption feature of the target species. The modulation frequency is .OMEGA.. As the light beam propagates through a sample, absorption by the target species converts some of the wavelength modulation into amplitude modulation (AM) of the light. When the light impinges onto a photodetector such as a photodiode, the output signal from the detector contains AC components at the modulation frequency, .OMEGA., and its higher harmonics, 2.OMEGA., 3.OMEGA., 4.OMEGA., etc. One of the AC components is selected for measurement using a phase sensitive detector such as a lock-in amplifier or a mixer. This signal processing step is known as demodulation and the signal obtained by demodulation at frequency n.OMEGA. is known as the nf signal. Usually a portion of the modulation waveform is used to generate a reference waveform (local oscillator) for the demodulator. The demodulated signal is related to the optical absorbance and to the intensity of the light beam.
Detailed theory describing WMS and the relationships between the absorption lineshape and demodulated lineshapes is given by Silver, "Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods," Applied Optics 31, 707-717(1992). In qualitative terms, the waveform produced by slowly stepping the average laser wavelength across an absorption line while demodulating at frequency n.OMEGA. looks like the nth derivative of the absorption lineshape.
The shape of a wavelength modulated spectrum depends strongly on the ratio of the extent of the wavelength modulation to the linewidth of the absorption feature. Any phenomenon that changes the absorber linewidth, such as variations in sample pressure or, to a lesser extent, variations in sample temperature, will change the shape and peak intensities of the corresponding wavelength modulation spectrum. Changes in absorber linewidth can, therefore, introduce error into quantitative applications of WMS. The present invention provides method and apparatus that improve WMS by reducing the measurement uncertainty resulting from such changes.
A number of methods exist that can be used to correct wavelength modulation spectra for changes in the absorber linewidth; each of these approaches, however, has some substantial limitation. For example, Wilson, "Modulation broadening of NMR and ESR line shapes," J. Appl. Phys. 34, 3276-3285 (1963), showed that the exact shape of a wavelength modulation spectrum ran be used to extract the absorber linewidth and, thereby, calculate the actual optical absorbance and the species concentration. Wilson's method, however, requires large signal-to-noise ratios in the WMS measurements in order to obtain accurate linewidths, absorbances, and species concentrations.
Goldstein, et al., have developed an improvement to wavelength modulation spectroscopy in which the detector signal at twice the modulation frequency (2.function.) is monitored while the extent of the wavelength modulation is changed. Goldstein, et al., "Gaseous Species Absorption Monitor," U.S. Pat. No. 5,026,991; and Goldstein, et al., "Measurement of molecular concentrations and line parameters using line-locked second harmonic spectroscopy with an AlGaAs diode laser," Appl. Opt. 31, 3409-3415 (1992). The response of the 2.function. signal is representative of the shape and width of the absorption line. Goldstein, et al.'s invention is simple to implement since it requires only a minor modification to standard WMS instrumentation. The most significant limitation of the invention, however, arises because lasers often respond non-linearly to applied modulation waveforms. Both the extent (depth) of modulation and the time dependence of the output wavelength may not track well the changes in the applied modulation signal. Proper implementation of the invention may require careful calibration of the response of each laser or using customized (i.e., non-sinusoidal) modulation waveforms. The non-linearities are particularly important when relatively large wavelength excursions are needed, such as occur for detecting absorbances from samples at atmospheric or higher pressure or from samples at high temperatures.
Species concentrations inferred from wavelength modulation spectra can be corrected by measuring sample temperature and pressure, and using corrections calculated from basic theory or from tabulated calibrations. The computational approach can be slow, however, and requires a significant amount of computing power, tabulating a set of corrections requires a lengthy and tedious calibration. In both cases, the instrument is made more complex and more expensive by adding pressure and temperature sensors.
Some of the demodulation and signal processing techniques employed by the present invention are known in the field of phase fluorometry. Berndt et al., "Fluorometry method and apparatus using a semiconductor laser diode as a light source," U.S. Pat. No. 5,196,709; Lakowicz et al., "Method and apparatus for multi-dimensional phase fluorescence lifetime imaging," U.S. Pat. No. 5,485,530; and Thompson et al., "Phase fluorometry using a continuously modulated laser diode," Anal. Chem. 64, 2075-2078 (1992). Demodulation of high frequency signals makes possible measurement of the phase lag, or time delay, between excitation of a fluorophore and fluorescence emission. These measurements can be used to determine fluorescence lifetimes, including deconvolution of multiple exponential decay rates (lifetimes). Phase fluorometry differs significantly from the present invention in several key ways: 1) the present invention measures optical absorption, not fluorescence; 2) the present invention is best performed using continuous wave (cw) light sources, whereas phase fluorometry requires amplitude modulated or pulsed light sources; 3) phase fluorometry relies on the amplitude modulation of the light source having many frequency components--effected either by varying the frequency of a sinusoidal amplitude modulation or by using short pulses--whereas wavelength modulation spectroscopy is best performed using a single modulation frequency, and relies on optical absorption to introduce multiple amplitude modulation frequencies onto a wavelength-modulated light beam; 4) the present invention provides information about absorption line shapes, whereas phase fluorometry typically uses a fixed wavelength source and acquires no information about spectral profiles.
The following patents likewise do not teach the present invention nor its objects and advantages:
Wong, "Concentration Detector," U.S. Pat. No. 5,047,639 describes an improvement to wavelength modulation spectroscopy which is useful for quantitave detection of a selected gas or gases using a diode laser. Wong describes 2.function. detection for gas concentration determinations. He employs 1.function. detection for line-locking, and describes using the 2.function. signal as a method for guaranteeing "capture" by the 1.function. error signal. However, Wong uses homodyne, not heterodyne demodulations; requires phase adjustments for all demodulations; and cannot acquire line width information.
Whittaker et al., "Method and Apparatus for Reducing Fringe Interference in Laser Spectroscopy," U.S. Pat. No. 5,267,019, also describes an improvement to wavelength (or frequency) modulation spectroscopy for use in detecting gases using diode laser;. Whittaker '019 achieves absorbance measurements with a reduction in unwanted optical interference fringes (etalons) by modulating the laser wavelength at two frequencies simultaneously and by performing sequential demodulations. While unwanted optical interference fringes can occur during practice of the present invention (and can be seen clearly in the phaseless WMS spectra shown in FIGS. 3, 4, and 6), the present invention does not address the fringes, nor would one skilled in the art expect the present invention to maximize or minimize the magnitudes of such fringes. Whittaker '019 is distinct from the present invention because he uses homodyne, not heterodyne demodulations; requires phase adjustments for all demodulations; cannot acquire line width information; and the demodulated lineshapes obtained (Whittaker '019 FIGS. 5-8) are significantly distorted relative to standard harmonic lineshapes, which greatly complicates quantitative measurement of absorbance and species concentrations.
Whittaker et al., "Method and Apparatus for Dual Modulation Laser Spectroscopy," U.S. Pat. No. 5,636,035, also describes an improvement to wavelength (or frequency) modulation spectroscopy for use in detecting gases using diode lasers. Whittaker '035 achieves absorbance measurements and laser line-locking by modulating the laser wavelength at two frequencies simultaneously and by performing two sets of sequential demodulations. Whittaker '035 is distinct from the present invention in that he uses homodyne, not heterodyne demodulations; requires phase adjustments for all demodulations; and cannot acquire line width information.
Zybin et al., "Spectroscopic Method with Double Modulation," U.S. Pat. No. 5,640,245, describes an improvement to optical spectroscopy that is best suited to laser light sources, particularly to diode lasers. He performs a wavelength or amplitude modulation of the laser beam at one frequency, f.sub.1, then modulates some optical property of the target species at a second, different frequency, f.sub.2. Improved detection limits are possible by demodulating the detector output at the sum frequency, f.sub.1 +f.sub.2 ; or at the difference frequency, f.sub.2 -f.sub.1 ; or at an integral harmonic of the sum or difference; or by using two demodulators in series with the first demodulator referenced to f.sub.1 and the second demodulator referenced to f.sub.2. Zybin's invention has relatively few applications, however, because it is usually difficult to find a straightforward method for modulating a useful optical property of the target species. Zybin's approach is most useful for detecting species in AC plasmas since species concentrations often vary synchronously with the plasma frequency. But, other applications listed by Zybin, such as changing the optical path length using "an electrically adjustable system of mirrors," are not feasible at the high frequencies, i.e., above 100 kHz, recommended in the description of the invention and, when implemented, may introduce other error sources such as unwanted optical interference fringes (etalons). More importantly, Zybin does not anticipate the present invention. As with Wong and with the two Whittaker patents, Zybin uses homodyne, not heterodyne demodulations; requires phase adjustments for all demodulations; and cannot acquire line shape information.
Lehmann, "Ring-Down Cavity Spectroscopy Cell Using Continuous Wave Excitation for Trace Species Detection," U.S. Pat. No. 5,528,040, describes a highly sensitive implementation of optical spectroscopy that is useful with diode lasers. His approach differs significantly from the instant invention and from the prior art cited above in that neither wavelength modulation nor frequency modulation techniques are used. Instead, weak optical absorbances are detected by using a special, mirrored optical cell that permits optical path lengths in excess of 1 km from a structure that is only about 1 meter long. Since Lehmann performs direct optical absorbance measurements, his invention can provide line width (and line strength) information, yet nothing in Lehmann's patent discloses or obviates the present invention. Key differences between Lehmann's approach and the present invention include: The present invention does not require a special optical cell, and can be used for open path or in situ measurements; and Lehmann does not require any modulation of laser wavelength nor demodulation of the detector output.
McCaul et al., "Gas Spectroscopy," U.S. Pat. No. 5,491,341, describes an optical spectroscopy technique that is well suited to diode lasers for gas measurements. McCaul's only similarity to the present invention is that both provide line width information. The two approaches are distinct, however, in nearly every other detail. Specifically, McCaul measures the optical transmission through a gas at a plurality--usually five--laser wavelengths that are symmetrically disposed about the absorption peak. These measurements provide the peak absorbance, the line width, and the laser wavelength error. The product of the peak absorbance and line width is proportional to the target gas concentration independent of changes in the line width. The laser wavelength error is used to correct the laser wavelength so that the plurality of measurements remains symmetrically disposed about the absorption peak. Other key differences include: (1) The laser wavelength is not varied continuously, as it is in wavelength modulation or frequency modulation spectroscopies. Instead, a sequence of step changes in laser wavelength is employed to obtain the plurality of measurements. Step durations are on the order of 0.5 ms, corresponding to a change rate of about 2 kHz. In contrast, the present invention employs continuous modulation, typically at frequencies of 50 kHz and higher. (2) One result of using step changes in laser wavelength is that synchronous demodulation (either homodyne or heterodyne) is not possible. Instead, McCaul obtains high detection sensitivity using fast subtraction or fast ratioing circuitry to cancel common mode noise between two photodetectors.
Kenny et al., "Method and System for Examining the Composition of a Fluid or Solid Sample Using Fluorescence and/or Absorption Spectroscopy," U.S. Pat. No. 5,491,344, describes an optical method for measuring species concentrations that is far removed from our application. Most importantly, Kenny uses multiple fixed wavelengths to perform absorption and/or fluorescence spectroscopy. Neither wavelength modulation nor detector demodulation are contemplated. Furthermore, the wavelength spacings are typically four orders of magnitude larger than the line widths of the individual rotational lines probed in typical diode laser spectroscopy. This coarse line spacing makes it impossible to obtain line width information. Other key differences include: (1) Kenny uses laser pulses, not a continuous wave (cw) laser beam, which obviates the type of wavelength modulation employed in the present invention; (2) Kenny does claim absorption spectroscopy using incoherent (i.e., lamp) light sources, but only in conjunction with fluorescence measurements made simultaneously with a pulsed laser system; and (3) Kenny does not use a continuously tunable laser but rather a system that can provide a plurality of fixed wavelengths.
Silver et al., "Laser Absorption Detection Enhancing Apparatus and Method," U.S. Pat. No. 4,934,816, describes a method and apparatus for removing the effects of unwanted optical interference fringes. Such fringes can occur during practice of the present invention (and can be seen clearly in the phaseless WMS spectra shown in FIGS. 3, 4, and 6), but the present invention does not address the fringes, nor would one skilled in the art expect the present invention to maximize or minimize the magnitudes of such fringes. More importantly, nothing in Silver anticipates the phaseless WMS technique. Silver is applicable to a wide variety of laser absorption methods (including the present invention) yet places no constraints on the absorbance method used. Silver invokes wavelength modulation spectroscopy in the preferred embodiment only to the extent that it allows high sensitivity absorption measurements such that unwanted optical interference fringes are significant compared with measured absorbances. Silver does not, however, mention limitations of WMS such as the need for phase adjustment or the lack of line shape information.
Bomse et al., "Mass Spectrometric Apparatus and Method," U.S. Pat. 5,015,848, which is in the field of mass spectrometry, has no relation to the present invention except for presence of a common inventor, but is included for sake of completeness.